The present invention relates to a method for producing a film, sheet, or other element having a top coat. More particularly, it relates to a method for applying a top coat which provides a functional property to an element by virtue of the top coat being on the uppermost surface of the film.
Photographic materials, as well as many other types of film or sheet construction, have become increasingly sophisticated and require certain functional properties in addition to their basic photosensitivity to properly serve the users needs. A particular class of desirable functional properties are those which exist by virtue of a top coat which resides on the uppermost surface of a film or sheet such as a photographic element. The top coat results from a coating composition which is applied to the surface of the element to become the uppermost surface thereof. For example, in a photographic element, the top coat is often applied to the photosensitive layer of the photographic element.
The top coat coating composition generally includes dissolved or dispersed materials and a solvent. After being applied to the uppermost surface of the photographic element, the coating composition is dried so that essentially only the dissolved or dispersed materials, and sometimes a binder, remain. It is those remaining materials which impart the desired functional property to the photographic element. However, functionality will result only if some effective amount of the materials (hereinafter referred to as "surface functional materials") remain on the uppermost surface of the photographic element after the coating composition has been dried.
Top coats comprising surface functional materials are quite common. Examples include diffusion transfer top coats which contain nucleating particles (used, e.g., to form direct acting printing plates); matte surface top coats containing matte agents (used in graphic arts materials to allow vacuum drawdown for intimate contact between films); and antistatic top coats containing conductive materials.
In the case of diffusion transfer top coats, direct acting printing plates are produced by the formation of an oleophilic silver layer, generated by a diffusion transfer process, on the uppermost surface of the printing plate after exposure and development. See, e.g., U.S. Pat. No. 4,361,635. Nucleating particles, composed of palladium specks or other materials as described in U.S. Pat. No. 4,298,673, are required to be on the uppermost surface of the diffusion transfer element to serve as sites for the formation of the oleophilic silver layer during development. If these particles are not physically present on the surface, they cannot serve as centers for silver development on the uppermost surface of the printing plate. As a result, an oleophilic, ink receptive surface will not form on the uppermost surface of the printing plate.
Matte surface top coats are used, for example, in the graphic arts industry. Contact exposures are made to reproduce and modify images that will eventually lead to a printing plate. In the process of making these contact exposures, an imaged film, a photosensitive element, a proofing element, and/or other graphic arts elements need to be drawn into intimate contact in a vacuum device. The intimate contact is required to assure good reproduction without spreading of the image. Image spreading occurs when there is a space, caused by trapped pockets of air, between the imaged film and the other element. Such air pockets are difficult to remove and lead to extended vacuum drawdown times. Additionally, Newton's rings, which are unwanted lines caused by light diffraction due to reflections off of adjacent surfaces, can occur. Both air pockets and Newton's rings are caused by the smooth texture of the imaged film and the other elements. As is known, such problems can be overcome by applying a matte surface top coat to either or both of the imaged film and the other element. When placed into contact with one another, the relatively rough matte surfaces allow the elements to be drawn into intimate contact without air pockets or Newton's rings, thereby permitting the vacuum drawdown to proceed in a reasonable time.
Typical matte surface top coats include matte agents which are particulates composed of silica or polymer having a size ranging from 2 to 10 microns. A coating composition containing the matte agent is typically applied to the uppermost surface of the undercoating (which contains the photosensitive layer of the photosensitive element) so that the matte surface top coat will form the uppermost surface of the photosensitive element. In order to be effective, the matte agent must cause irregularities to form on the uppermost surface of the photosensitive element after the coating composition has been dried.
An added concern with matte surface top coats is a phenomenon known as "starry night." As the photosensitive undercoating and top coat are being dried, the matte particles can be forced into the undercoating by the surface tension forces developed during drying and displace silver halide particles in the undercoating, thereby causing voids to appear in the image. Thus, it is desirable to keep the matte agent on the uppermost surface of the photographic element for this additional reason.
A problem which is common to top coats containing dispersed or dissolved surface-functional materials, such as diffusion transfer top coats or matte surface top coats, is the migration of the surface-functional material into the undercoating. When this occurs, the surface-functional material becomes incapable of, or less effective in, performing its desired function since functionality is dependent upon the physical presence of the material on the uppermost surface of the photographic element. In addition, with matte surface top coats in photographic elements containing silver halide emulsions, such migration can also cause the starry night effect.
The undercoating (i.e. the coating(s) or sublayer(s) located between the top coat and the support for the element and, in the case of photographic elements, containing the photosensitive portion of the element) includes a binder, such as gelatin. Typically, the solvent used in the top coat, e.g., water, is "compatible" with the binder. That is, the solvent is capable of penetrating the binder and causing it to swell. While some swelling is desirable in that it promotes adhesion of the top coat to the undercoating, swelling is also believed to be a leading contributor in the migration of the dispersed or dissolved materials from the top coat and into the binder, thereby causing the surface-dependent top coat to become non-functional since it is no longer on the uppermost surface of the element.
Conventional solutions to this problem include the addition of hardening compounds into the undercoating prior to the application of the top coat thereto. Typical hardening compounds such as, e.g., formaldehyde or mucochloric acid, begin to cross-link upon drying of the undercoating. By cross-linking, hardening compounds have the effect of reducing the degree to which the binder in the undercoating can swell, thereby reducing the migration of the dissolved or dispersed elements from the top coat into the undercoating.
Unfortunately, most hardening compounds cross-link at a rate which is too slow to permit successful in-line application of the top coat composition to the photographic element. Typically, the undercoating containing the binder and hardening compound is applied to a support which is in the form of a continuous web. After the undercoating is applied to the support, the coated support must be wound into a roll, removed from the coating apparatus, and stored for a period of time to allow the hardening compound to cross-link. This wind-up/hardening period must be long enough to permit a sufficient degree of cross-linking by the hardening compound to impart enough swelling resistance to the binder in the undercoating so that the dissolved or dispersed surface-functional materials in the top coat are prevented from migrating into the undercoating. Typically, such a period can range from one hour to one week. In either case, after the hardening period has expired, the photographic element is removed from storage and re-inserted into the coating apparatus so that the top coat can be applied to the hardened undercoating. This two-stage procedure is both time consuming and costly, as well as being highly inconvenient.
An alternative approach to the two-stage procedure described above has been proposed in PCT Publication Number WO 92/15921. That reference provides a method for accelerating the hardening of a photographic coating composition containing a binder and a hardener, where the coating composition has been coated on a continuous web-like support. In addition to a chill section and a drier (which are normally used in such processes), the method employs a tempering zone, a high-temperature heating zone, an afterhardening reaction and incubation zone, a post cooling zone, a moisture content-adjusting zone, and a cooling zone. After the support has been coated and sent through the chill section and drier, the coated support is transported through the various zones in the order listed above. Through precise control of temperature, relative humidity, and air flow, especially in the reaction and incubation zone, the coating is said to be 85% hardened before exiting the coating machine. Moreover, the duration of time in the reaction and incubation zone is said to be less than 10 minutes. The disadvantage of such a method, however, is the capital, maintenance, and operational costs for each of the aforementioned zones.
Accordingly, it is seen that a need exists in the art for a method of producing a multilayered photographic element having an undercoating and surface-functional top coat in which the top coat is applied in the same continuous process stream as the undercoating is applied, and in which the top coat does not migrate into the undercoating to the extent that it is incapable of providing a desired surface-related functional property to the multilayered element.